Life cycle, growth and diet of Balston's pygmy perch in its natural habitat of acidic pools in south-western Australia

Nannatherina balstoni is found in a few acidic pools (pH 3.9–6.0) in the extreme south-western corner of Australia. Although many of these pools become dry during summer and early autumn, they are recolonized by fish from nearby pools that overflow during winter floods. N. balstoni spawns at the end of its first year of life, when, on average, the males and females have reached 60 and 63 mm t.l ., respectively, and then usually die within the next few months. The largest fish, which was one of only three in its third year of life, measured 90 mm and weighed 7.3 g. The von Bertalanffy growth curve parameters for L₀, K and t₀ were 71.2 mm, 1.69 and – 0.078 for males and 82.6 mm, 1.31 and – 0.095 for females. Fecundity ranged from 550 to 1600. N. balstoni. spawns during the middle of winter, after heavy flooding and when water temperatures are at, or close to, their annual minima. This enables the larvae and young juveniles to capitalize on those aquatic organisms, especially Cladocera, which are very abundant amongst the flooded riparian vegetation that surrounds the pools in winter and spring. Hence, the fish grow rapidly and attain an appreciable size before summer, when the increases that occur in the densities of the larger carnivorous fish species, as a result of marked declines in water levels, increases the chances of predation. By spring, when most N. balstoni exceed 25 mm t .l ., the diet changes markedly to one that consists almost exclusively of terrestrial fauna. In contrast, three of the six co-occurring native species of teleost feed on aquatic and terrestrial fauna throughout the year, while a further two feed only within the water column and the sixth feeds on benthic invertebrates. The pronounced shift in diet exhibited by N. balstoni as it increases in size, allied to dietary differences amongst the other six co-occurring species, reduces any potential for interspecific competition for food resources during the summer and autumn, when such resources are declining.